Is exit selection necessary in D3?

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Icreated this simple example using data join in D3. It works: when user clicks on +1 a green is added to the list, when clicks on -1 the last color in the list is removed.

Here the code:

const colors = ["white", "yellow", "red", "brown", "orange"];
    const root = d3.select("#root");
    const addColor = d3.select("#add-color");
    addColor.on("click", (d) => {
      colors.push("green");
      update();
    });
    const removeColor = d3.select("#remove-color");
    removeColor.on("click", (d) => {
      colors.pop();
      update();
    });
    const ul = root.append("ul");
    ul.selectAll("li")
      .data(colors)
      .join(
        (enter) => enter.append("li").text((d) => d),
        (update) => update,
        (exit) => exit.remove()
      );
      
    function update(){
      ul.selectAll("li")
      .data(colors)
      .join(
        (enter) => enter.append("li").text((d) => d),
        (update) => update,
        (exit) => exit.remove()
      );
    }
#buttons-container {
      display: flex;
      margin-bottom: 30px;
    }
    #buttons-container div {
      min-width: 30px;
      text-align: center;
      cursor: pointer;
      border: 1px solid black;
      margin-right: 50px;
    }
<!DOCTYPE html>
    <meta charset="utf-8" />
    <html>
      <body>
        <div id="root">
          <div id="buttons-container">
            <div id="add-color">+1</div>
            <div id="remove-color">-1</div>
          </div>
        </root>
        <script
          type="text/javascript"
          src="https://cdnjs.cloudflare.com/ajax/libs/d3/6.5.0/d3.min.js"
        ></script>
        <script type="text/javascript" src="./index.js"></script>
        <script type="text/css" src="./styles.css"></script>
      </body>
    </html>

Then I commented the exit action:

function update() {
  ul.selectAll("li")
    .data(flavors, (d) => d)
    .join(
      (enter) =>
        enter
          .append("li")
          .text((d) => d)
          .style("color", "green"),
      (update) => update.style("color", "steelblue"),
      //(exit) => exit.style("color", "red").remove()
    );
}

And it works in the same way. What am I missing? Is exit necessary?

115 thoughts on “Is exit selection necessary in D3?”

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